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Environment / Sustainability

Learning from Colorado flash floods

July 27, 2009

What have we learned from our deadly Colorado flash floods in the past 33 years? July 31 is the 33rd anniversary of Colorado's Big Thompson flood, which took more than 140 lives, and July 28 was the 12th anniversary of the Fort Collins flood when five people were killed along Spring Creek.

Unwarned and vulnerable

Since the 1976 Big Thompson flood, early warning systems have advanced and now include emergency call-back systems, Doppler radar, satellite imagery, and automated stream and rain gauge networks. The emphasis on the “detection” rather than “response” part of the warning system, however, does not solve many of the problems identified in 1976.

(Photo: A warning sign at the entrance of Big Thompson Canyon.)

Unfortunately, flash floods often occur in catchments too small for the rainfall signal to appear on Doppler radar, leaving the public unwarned and vulnerable. Are campers today more aware that a severe thunderstorm can produce catastrophic flash flooding in the middle of the night? Probably not.

Difficult to warn non-residents about flash floods

Even if rain can be detected on radar, it is still difficult to notify campers and other non-residents (e.g., tourists) about short-fuse flash floods. Cell phone reception is not very good in mountain canyons. Publicizing information about flash flood recurrence intervals is also problematic — the recurrence interval for the Big Thompson River varies from 500 to 10,000 years in the scientific literature. Experts call for a comprehensive integration of social and natural sciences to improve the understanding of public responses and target loss reduction.

Flash floods are characterized by their sudden onset and fast and violent movement. They are particularly difficult to forecast accurately, and they leave very little lead time for warnings. Flash floods can surprise people who are in the midst of their daily activities, and they have particularly severe consequences for people who travel across flooded roads.

50 percent of flash flood fatalities are vehicle-related

Several studies show that a large number of flood-related deaths occur among motorists on the road, and in the United States 50 percent of flash flood fatalities are vehicle-related.

(Photo: Vehicles were left stranded in the aftermath of the 1976 Big Thompson flood. Courtesy of Water Resources Archive.)

In 1976, over 140 people were killed on the night of Colorado’s Big Thompson flood. Of the approximately 2,500 folks who were in the canyon, which is located west of Loveland, most received no official warning that a catastrophic flash flood was about to shatter their lives. Seven of the victims reacted by “doing the right thing” — aware of the flood’s approach, they immediately moved to higher ground. However, they then miscalculated the flood’s actual moment of arrival and returned to lower ground to move a vehicle or collect something from a dwelling. They paid for that decision with their lives.

Behaviors that led to survival in Big Thompson disaster

Eve Gruntfest studied the Big Thompson disaster in hopes of deriving lessons for the many officials along Colorado’s urban front range who realized that the 14 inches of rain in less than three hours could have similarly affected them.

In 1976, flash floods were not recognized as a separate category of flood, but much has changed in 33 years. Gruntfest’s geography thesis focused on the behavior of people during the Big Thompson flood. She identified which activities led to survival and which did not. Clearly, climbing to safety was the best action to take, and those who stayed in a car or did nothing were more likely to die in the flood.

Learning from analyses of actual behavior

Most flash flood warning research, including the National Science Foundation project Gruntfest recently completed, focuses on public perceptions of warnings rather than analyses of actual behavior. After 33 years, Gruntfest’s current work is based on human response, and she and her colleagues are confident they can learn more from what people do than from what they say they would do.

(Photo: The Big Thompson flood left a house precariously teetering, a quarter of a mile below Glen Comfort. Courtesy of USGS, photo by R.R. Shroba.)

Recent investigations are leading to new ways of understanding warning response; for example — finding out what motivates drivers to change their routes or the timing of their trips in light of potentially deadly high water along their regular route.

Examining behavioral patterns in response to warnings and to flash flood conditions involves asking several questions.

What does it take to change driver behavior when faced with flooded roads?

How can we improve warning messages to convey our knowledge with the longest lead time and most geographic specificity?

Can warnings be issued for specific low-water crossings rather than for counties or polygons?

No "one-size-fits-all" effective flood warnings

In summary, there is no “one-size-fits-all” answer to more effective flood warnings. People need different information at different times of the day, which presents a serious challenge for weather forecasters who shoulder warning responsibilities.

By building on traditional flash flood research by meteorologists and hydrologists that focuses on rainfall, streamflow, radar, and satellite forecasts, collaborations between social scientists and practitioners from the National Weather Service offer potential for substantially reducing public vulnerability to flash flooding.

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Excerpt from article published in Colorado Water, the newsletter of the Water Center of Colorado State University, July/Aug. 2009, Vol. 26, Issue 4.